1. Field of the Invention
The present invention relates to a device and a method for coding data, a device and a method for decoding data and communication devices using them.
This invention is of use in all areas of coding, storage and transmission of digital data, and in particular those using an alphabet in which the number of symbols is different from 256.
In particular, the present invention applies to transmission of sextuples, modulated by an amplitude modulation in accordance with two carriers in quadrature (hereinafter called xe2x80x9cQAMxe2x80x9d) with 64 states (hereinafter called xe2x80x9cQAM-64xe2x80x9d).
2. Description of the Prior Art
There are many coding methods allowing error correction of digital data. Among the best known codes used at present, the Reed-Solomon codes may be mentioned. These constitute a powerful means of correcting data transmission errors. They may be constructed on any alphabet containing a number of symbols which is equal to a power, pm, of a prime number, p.
Very often a value of m equal to 8 and a value of p equal to 2 is chosen. The consequence of this large predominance of codes on alphabets with 28 (=256) symbols is that the majority of Reed-Solomon coders and decoders which are found on the market work on this alphabet. Their low relative cost and their high efficiency means that they are used in many areas notably in the transmission or storage of digital data on tape or disc. This is because a Reed-Solomon coder or decoder constructed to work on 28 symbols can also work on an alphabet containing 24 (=16), 22 (=4) or 21 (=2) symbols. The corresponding codes are commonly known under the name xe2x80x9cBCH codesxe2x80x9d on respectively GF(24), GF(22) or GF(2) (where GF means xe2x80x9cGalois Fieldxe2x80x9d). Nevertheless, an alphabet with 64 symbols cannot be treated in this way because the Galois field GF(26) is not a sub-field of GF(28).
Therefore, when the natural alphabet of an application contains 64 symbols, as in a system using a QAM-64 modulation, these symbols cannot be considered as words of a code on GF(28).
Consequently, in the case of transmission of data modulated with a QAM-64 modulation, a person skilled in the art of transmission wishing to use inexpensive Reed-Solomon coding components uses them in a non-optimal way: he considers a sequence of binary data as a flow of octets which he codes with a Reed-Solomon coder. The code words produced are considered with no particular care as a sequence of 6-uples; each 6-uple is finally modulated in the form of a QAM-64 symbol.
On receipt, each symbol received is interpreted as a binary 6-uple. The resulting sequence of binary data is considered as a sequence of octets specifying one GF(28) element. This sequence of GF(28) elements, entering a Reed-Solomon decoder corresponding to the coder used at transmission, will be decoded in an ordinary manner. This manner of formatting QAM-64 symbols in octets has a significant drawback. As in any transmission system, transmission errors occur on QAM-64 symbols. However, the 6 bits of the same QAM-64 symbol may have been coded over two consecutive octets. As the Reed-Solomon decoder works on octets, it is possible that an error on a single QAM-64 symbol could produce an error on two consecutive octets, which amounts to doubling the error affecting the data transmitted in this manner. This reduces the correction capability of a Reed-Solomon coder expressed as a number of correctable QAM-64 symbols.
In order to solve the problem disclosed previously, a code specified on GF(26) could be chosen. Two other problems then arise:
on the one hand, in this case, a component of this type is not readily found today on general sale, and
on the other hand, if it is wished to use words of length greater than or equal to 64 binary 6-uples, no Reed-Solomon code of this length is known on GF(26).
Consequently, the redundancy of the codes is used less efficiently; for a given power of correction, a greater redundancy is required (in other words, the efficiency of the code is lower).
The present invention intends to remedy these drawbacks. It aims, above all, to allow the transmission of symbols forming part of an alphabet having a number of symbols equal to the alphabet used by a transmission means, while coding the said symbols with a code adapted to another alphabet.
To this end, the invention relates, according to a first aspect, to a device for transmitting digital data to be transmitted, representing a physical quantity and belonging to a first alphabet having P symbols, characterized in that it has:
a labelling means adapted to map each symbol of the first alphabet to secondary digital data belonging to a second alphabet having Q symbols, with Q strictly greater than P, P symbols of the second alphabet each representing exclusively one and only one symbol of the first alphabet,
a coding means adapted to determine redundant data belonging to the second alphabet, using predetermined coding rules taking into account the secondary digital data, and
a transmission means adapted to modulate at least one physical quantity into a series of signals each capable of taking a number P of different values, and successively representing, according to predetermined transmission rules, on the one hand, the digital data to be transmitted and, on the other hand, the said redundant data.
Correlatively, the invention relates, according to a second aspect, to a device for receiving signals representing so-called xe2x80x9cto be transmittedxe2x80x9d digital data belonging to a first alphabet having P symbols, characterised in that it has:
an identification means adapted to receive received digital data belonging to an alphabet having P symbols, and to map received digital data to so-called xe2x80x9cto be decodedxe2x80x9d symbols belonging to an alphabet having Q symbols, with Q strictly greater than P,
a decoding means adapted to correct errors affecting the symbols to be decoded, using predetermined decoding rules taking into account so-called xe2x80x9credundantxe2x80x9d symbols to be decoded, and to supply so-called xe2x80x9ccorrectedxe2x80x9d symbols, and
a translation means adapted to supply so-called xe2x80x9ctransmittedxe2x80x9d digital data capable of taking a number P of different values, and successively representing, according to predetermined so-called xe2x80x9ctranslationxe2x80x9d rules, corrected symbols.
Correlatively, the invention relates, according to a third aspect, to a method for transmitting digital data to be transmitted, representing a physical quantity and belonging to a first alphabet having P symbols, characterized in that it has:
a labelling step during which each symbol of the first alphabet is mapped to secondary digital data belonging to a second alphabet having Q symbols, with Q strictly greater than P, P symbols of the second alphabet each representing exclusively one and only one symbol of the first alphabet,
a coding step during which redundant data belonging to the second alphabet are determined, using predetermined coding rules taking into account the secondary digital data, and
a transmission step during which at least one physical quantity is modulated into a series of signals each capable of taking a number P of different values, and successively representing, according to predetermined transmission rules, on the one hand, digital data to be transmitted and, on the other hand, the said redundant data.
Correlatively, the invention relates, according to a third aspect, to a method for receiving signals representing so-called xe2x80x9cto be transmittedxe2x80x9d digital data belonging to a first alphabet having P symbols, characterized in that it has:
an identification step during which digital data belonging to an alphabet having P symbols are received, and received digital data are mapped to so-called xe2x80x9cto be decodedxe2x80x9d symbols belonging to an alphabet having Q symbols, with Q strictly greater than P,
a decoding step during which errors affecting the symbols to be decoded are corrected, using predetermined decoding rules taking into account so-called xe2x80x9credundantxe2x80x9d symbols to be decoded, and to supply so-called xe2x80x9ccorrectedxe2x80x9d symbols, and
a translation step during which so-called xe2x80x9ctransmittedxe2x80x9d digital data capable of taking a number P of different values are supplied, successively representing, according to predetermined so-called xe2x80x9ctranslationxe2x80x9d rules, corrected symbols.
By virtue of these provisions, the coding means used, which works on the second alphabet, may be of a lower cost than a coding means working on the first alphabet.
Furthermore, the symbols transmitted by the transmission means represent, at least partially, digital data of the first alphabet, which makes it possible to not transmit, to represent them, symbols of the second alphabet. In this way, the invention makes it possible to optimize the number of elementary symbols, that is to say generally the binary symbols, which represent both the data to be transmitted and the redundant symbols. The invention thus makes it possible to increase the efficiency of the transmission.
According to preferential characteristics of the first aspect of the invention:
the labelling means is adapted to add predetermined digital data to each symbol of the first alphabet in order to form the secondary digital data belonging to the second alphabet which correspond to the symbol of the first alphabet, and advantageously
the labelling means is adapted to add identical digital data to each symbol of the first alphabet in order to form the secondary digital data belonging to the second alphabet which correspond to the symbol of the first alphabet.
Correlatively, according to preferential characteristics of the second aspect of the invention:
the translation means is adapted to receive corrected symbols each capable of being represented by a sequence of binary data and to supply transmitted digital data items each representing segments of the sequences and advantageously
the translation means is adapted to remove predetermined digital data from each corrected symbol in order to form a transmitted digital data item.
Correlatively, according to preferential characteristics of the third aspect of the invention:
during the labelling step, predetermined digital data are added to each symbol of the first alphabet in order to form the secondary digital data belonging to the second alphabet which correspond to the symbol of the first alphabet and advantageously
during the labelling step, identical digital data are added to each symbol of the first alphabet in order to form the secondary digital data belonging to the second alphabet which correspond to the symbol of the first alphabet.
Correlatively, according to preferential characteristics of the fourth aspect of the invention:
during the translation step, corrected symbols each capable of being represented by a sequence of binary data are received and transmitted digital data items each representing segments of the sequences are supplied and advantageously
during the translation step, predetermined digital data are removed from each corrected symbol in order to form a transmitted digital data item.
By virtue of these provisions, the labelling means is particularly easy to implement. It may, in fact, be composed only of electrical connections maintained at a constant potential, for example at ground, these connections representing the added digital data.
According to preferential characteristics of the first aspect of the invention, the transmission device as briefly explained above has a preselection means adapted:
to receive, from the coding means, the redundant symbols which belong to the alphabet having a number Q of symbols, and
to supply digital data to the transmission means.
Advantageously, the preselection means is adapted to receive redundant symbols each capable of being represented by a sequence of binary data and to supply digital data items each representing segments of the said sequences.
Correlatively, according to preferential characteristics of the third aspect of the invention, the transmission method as briefly explained above has a preselection step during which:
following the coding step, the redundant symbols which belong to the alphabet having a number Q of symbols are received, and
digital data are supplied which are then processed during the transmission step.
Advantageously, during the preselection step, redundant symbols each capable of being represented by a sequence of binary data are received and digital data items each representing segments of the sequences are supplied.
By virtue of these provisions, the preselection means is particularly easy to implement.
According to preferential characteristics of the first aspect of the invention, the transmission device as briefly explained above has a mapping means adapted to map, according to a set of predetermined so-called xe2x80x9cmappingxe2x80x9d rules, each digital data couple selected by the selection means to an amplitude couple, the transmission means being adapted to perform the transmission of the signal in quadrature, with its two components being respectively modified by the first and second amplitudes of the said amplitude couples.
Advantageously, the set of predetermined mapping rules includes at least the first rule according to which, when an estimated probability that two amplitude couples are confused (i.e., are mistaken) after the transmission is greater than a first predetermined value, then the digital data couples corresponding to the two amplitude couples have first or second digital data items of the same value.
By virtue of these provisions, the most probable errors affecting the components of a signal transmitted by the transmission means affect only one of the digital data items of the digital data couple which corresponds to this signal.
Thus, even when these digital data do not have any redundancy capable of allowing the detection or correction of certain of the errors affecting them, the invention makes it possible to reduce the consequences of these errors.
When these digital data have redundant items capable of allowing the correction of errors affecting signal components, the invention makes it possible:
to reduce the number of redundant items necessary to correct a given number of errors,
to increase the number of errors which can be corrected by the use of a given number of redundant items, and
to increase the efficiency of the transmission.
In general, each redundant item makes it possible to correct one error affecting a digital data item. By virtue of the invention, each redundant item makes it possible, for the most probable errors, to correct at least one error affecting a signal (and more than one error, when the digital data items corresponding to two signals are connected and therefore corrected simultaneously).
According to particular characteristics of the first aspect of the invention:
the preselection means is adapted to receive redundant symbols each capable of being represented by a sequence of an even number of binary data items and to supply digital data items each representing half of the binary data items of the sequence, and/or
the set of predetermined mapping rules includes at least the rule according to which, when an estimated probability that two amplitude couples are confused after the transmission is greater than a first predetermined value, then one of the digital data couples corresponding to the two amplitude couples does not represent any digital data coming from the preselection means.
Correlatively, according to particular characteristics of the third aspect of the invention:
during the preselection step, redundant symbols each capable of being represented by a sequence of an even number of binary data items are received and digital data items each representing half of the binary data items of the sequence are supplied and/or
the set of predetermined mapping rules includes at least the rule according to which, when an estimated probability that two amplitude couples are confused after the transmission is greater than a first predetermined value, then one of the digital data couples corresponding to the two amplitude couples does not represent any digital data processed during the preselection step.
By virtue of these provisions, the most probable errors affecting the components of a signal transmitted by the transmission means do not affect any of the redundant symbols.
The present invention thus makes it possible to optimise, for a given coding efficiency, the error detection and correction power of error correction components (for example Reed-Solomon codes) of low cost, in particular working on GF(256), in the case of a transmission modulated with a QAM-64.
Furthermore, it should be noted that lengths of code words, that is to say in this case redundant symbols, greater than 63 6-uples can be used.
Thus the symbols which can be represented by a single component couple of a signal can benefit from a coding performed in an alphabet of which all the symbols cannot be represented by a single component couple of a signal.
The invention also relates to a network characterized in that it has a transmission medium, at least one transmission device as briefly disclosed above and at least one receiving device as briefly disclosed above, the signals transmitted by the transmission means of the transmission device being received, possibly affected by errors, by the receiving means of the receiving device.
The advantages of this network are identical to those described above and are therefore not repeated here.